## 2.3. Scheduling Tasks

Like other flavors of Unix, Mac OS X uses cron to schedule tasks for periodic execution. Each user's cron jobs are controlled by configuration files that you can edit with crontab -e (to list the contents of the file, use crontab -l).

### 2.3.1. Default cron Jobs

The global crontab file is contained in /etc/crontab. It includes three cron jobs by default, which run the scripts contained in subdirectories of the /etc/periodic directory: /etc/periodic/daily, /etc/periodic/weekly, and /etc/periodic/monthly. Each of these directories contains one or more scripts:

>     /etc/periodic/daily/100.clean-logs
>     /etc/periodic/daily/500.daily
>     /etc/periodic/monthly/500.monthly
>     /etc/periodic/weekly/500.weekly

By default, /etc/crontab runs them in the wee hours of the night:

>     15 3 * * *       root    periodic daily
>     30 4 * * 6       root    periodic weekly
>     30 5 1 * *       root    periodic monthly

So, if your Mac is not usually turned on at those times, you could either edit the crontab file or remember to run them periodically using the following syntax:

>     sudo periodic daily weekly monthly

As you'll see in Chapter 3, it is vitally important that you run these jobs to ensure that your local NetInfo database is backed up.

You should not modify these files, because they may be replaced by future system updates. Instead, create a /etc/daily.local, /etc/weekly.local, or /etc/monthly.local file to hold your site-specific cron jobs. The cron jobs are simply shell scripts that contain commands to be run as root. The local cron jobs are invoked at the end of the 500.daily, 500.weekly, and 500.monthly scripts found in the /etc/periodic subdirectory.

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